The present invention relates to a system and a method for controlling an internal combustion engine.
In order to efficiently operate an internal combustion engine, it is important to achieve good control of the air-fuel ratio. This can be accomplished by determining the cylinder flow and adjusting the amount of fuel to be injected accordingly to achieve a desired air-fuel ratio. Therefore, it is important to obtain an accurate estimate of the cylinder flow. One method is described in a pending U.S. application Ser. No. 09/769,800 owned by the assignee of the present invention and incorporated herein by reference, which teaches an estimation algorithm for determining engine cylinder flow using both an airflow sensor (MAF) and an intake manifold pressure (MAP) sensor. This MAP-MAF estimation algorithm uses the information on the time rate of change of the intake manifold pressure signal to correctly estimate cylinder flow during transients, and precisely matches the MAF sensor measurement at steady state.
However, under some circumstances the MAF sensor reading may become less accurate, thus negatively affecting the overall accuracy of the cylinder flow estimate. For example, in systems where a hot wire-type MAF sensor is used, the sensor does not reach operating temperature immediately upon start-up of the engine. Therefore, it is possible for the MAF sensor reading to not be accurate for the first 30-60 seconds of engine operation. Additionally, at high throttle angles, pulsation and backflow may affect the accuracy of the MAF sensor reading. Therefore, under the circumstances where MAF sensor reading accuracy is reduced, other methods of estimating cylinder flow that are not dependent on the MAF sensor reading are required. One such system is described in U.S. Pat. No. 4,644,474 owned by the assignee of the present invention, wherein engine operating conditions are monitored to determine when to switch between the MAF sensor reading and the estimate of the airflow based on the speed-density equation.
While this system provides satisfactory results, the inventors herein have recognized that an improved performance can be achieved. Specifically, since there is always some difference between an estimated and an actual reading, or between two different types of estimates, switching between them may cause abrupt fluctuations in the air-fuel ratio and engine torque, thus degrading vehicle drivability, fuel economy, and emission control.
The present invention teaches a method for accurately estimating cylinder flow under all operating conditions while eliminating any fluctuations that may result due to switching between different types of estimates.
In accordance with the present invention, a method and system for estimating cylinder flow in an internal combustion engine include: calculating a first cylinder flow estimate based on a first algorithm; providing an indication of an operating condition; in response to said indication, calculating a second cylinder flow estimate based on a second algorithm; and adjusting said second cylinder flow estimate based on said first cylinder flow estimate for a predetermined period of time thereby providing a smooth transition between said first estimate and said second estimate.
An advantage of the present invention is that a more accurate method of estimating cylinder flow is achieved during all operating conditions, therefore resulting in improved air-fuel ratio control, and thus improved fuel economy, emission control and vehicle drivability.
Another advantage of the present invention is that it results in a smooth transition between the two types of estimates, and therefore eliminates abrupt torque fluctuations and improves driver satisfaction.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.